There are a number of different classes of rotary trimmer heads, defined generally by the manner in which the trimmer line is fed, spooled or replaced in the trimmer head and/or by how the trimmer line is discharged, indexed or lengthened during use of the trimmer head. For example, “fixed-line” trimmer heads are one such class and must be manually loaded with regards to line replacement. As the line wears; the consumer manually inserts a new length of trimmer line into a clamp or tortuous line channel, and there is no indexing or lengthening of the line during use. The majority of aftermarket trimmer heads for rotary trimmers that are sold are fixed-line trimmer heads because they are easily designed to be universally adaptable with respect to working with essentially any brand of rotary trimmer, regardless of the operation of the trimmer head or the direction of rotation, as will be explained below.
Another class of rotary trimmer heads is the “self-indexing” trimmer head. With this class of rotary trimmer head, there is only one force that will determine how trimmer line is discharged from the trimmer head—the air drag on the line. For this class of trimmer heads, the housing is connected to the rotating trimmer machine shaft. Self-indexing trimmer heads contain a mechanism to lock and unlock the relative rotation of the spool of trimmer line based upon centrifugal forces, which change the line length. As the line wears, there is less air drag and the rotational speed of the trimmer head increases. With increased rotational speed, the mechanism unlocks the spool. Trimmer line is released, and the trimmer head speed slows due to more air drag on the longer length of trimmer line. The mechanism locks the spool, due to less centrifugal force, and the process repeats. With self-indexing trimmer heads, there is no part that is bumped on the ground and the lengthened or indexing of the line, as the name of this class of trimmer head implies, is controlled entirely by the trimmer head itself. No manual feeding of a new trimmer line is required. This class of trimmer heads is commonly used with electric trimmers that use 50 and 65 mil trimmer line. Importantly, however, the indexing mechanism has to be designed to function with one size of line. Thus, while this works well for an original equipment manufacturer (“OEM”) that can control what trimmer line to be purchased by the user, it is often the case that the user may not want to be limited to that particular trimmer line. Unfortunately, aftermarket replacement of these trimmer lines is very difficult and limited by the size of the trimmer line and the rotation/operation of the trimmer line in either the clockwise or counterclockwise direction. Thus, this class of trimmer heads has not been popular with cost-conscious customers who want to use less expensive or larger size lines. Once the line runs out, it must be replaced with the OEM's designated anti size of trimmer line. Hence, if the OEM, which typically sells pre-wound spools of the designated trimmer line for this class of trimmer head, is not big enough or does not have a distribution channel that is large enough to allow the customer to easily obtain and afford to purchase the designated trimmer line, the purchaser of trimmers having this class of trimmer head will not be happy and, thus, this class of trimmer head has not been exceedingly popular.
A third class of trimmer heads is the “bump-feed” or “bump-activated” trimmer heads. Historically, a bump-activated trimmer head has been designed with a bump knob or similar ground contacting member that is mechanically linked to the internal spool in the rotary trimmer head such that both parts (i.e., the bump knob and the internal spool) had the same rotational speed. In doing so, the bump knobs have been known to wear to the point of being useless. For these historical designs, there are two (2) forces together that will determine how the line is discharged from the trimmer head. One, like the self-indexing trimmer head, is the air drag on the line. The second force is the difference in rotational speed between (a) the spool holding a reserve of trimmer line and (b) the housing containing an eyelet from which the line is discharged. The force from air drag helps to pull the line from the head, but it is a minor force in magnitude compared to the force generated due to difference in rotations speeds occurring once the bump knob impacts the ground. Bump-feed trimmer heads have a ground contacting member (called a bump knob) which is mechanically connected to the internal spool within the trimmer head so that the two always have the same rotation speed. When this bump knob contacts the ground for the purpose of releasing more line, the contact with the ground slows the rotational speed of the knob and the spool in concert. The bumping action creates a vertical upward force, lifting the spool upward in the housing. For this brief moment, the spool is no longer locked to the housing, and can rotate independent of the housing. The difference in the rotation speeds between the spool and the housing causes line to be released from or pulled into the head, depending on a) the direction of rotation and b) the winding direction of the line.
There are different types of trimmer heads within the class of bump activated trimmer heads; however, while some of these trimmer heads may be of more than one type. One type is defined as a “universal” bump-activated trimmer head because it is “universally adaptable” to all bump-activated trimmers. That is, a “universal” bump-feed trimmer head is one that can be utilized by a consumer regardless of whether the trimmer head rotates clockwise on the trimmer or rotates counterclockwise on the trimmer. It will be appreciated that the term “bump-feed” and “bump-activated” can be used interchangeably. Given that the market for replacement trimmer heads is driven by existing trimmers that rotate clockwise or rotate counterclockwise, universally adaptable trimmer heads sold as replacements are highly desirable. However, with all bump-feed trimmer heads heretofore, the trimmer head designer and manufacturer has to communicate to the consumer which way to wind the line onto the trimmer head, which is dependent upon which way the trimmer head rotates on the trimmer. This is relatively easy for an OEM, since the OEM already knows the rotational direction of the trimmer machine to which the head is to be connected. Thus, the OEM only needs to communicate the winding direction (i.e., either clockwise or counterclockwise) to the consumer. However, this issue is a communication nightmare for a company making aftermarket trimmer heads for multiple different trimmers. The aftermarket trimmer head manufacturer must communicate how to wind the trimmer line onto a trimmer head for both the clockwise and counterclockwise direction. The aftermarket manufacturer must also rely on the consumer to determine whether the trimmer head will rotate clockwise or counterclockwise on the trimmer as this will determine which way to wind the line, and then also rely on the consumer to wind the line properly based upon the instructions communicated.
This same communication nightmare is even more complicated for the next type of bump-activated trimmer heads, the “easy-load” or “easy-to-load” bump-activated trimmer heads. An easy-load bump-feed trimmer head is a relatively new type of bump-feed trimmer head that has emerged in just the last 15 years. In addition to trying to address the “communication nightmare” defined above, manufacturers have been trying to make winding the trimmer line onto the spool easier for the consumer; hence, the name “easy-load.” The challenges of winding trimmer line onto the internal spool and then threading the ends of the line through the eyelets while inserting the spool into the housing have been well documented in the commercial and patent literature. This new class of trimmer head allows line to be loaded into the trimmer head without disassembly of the spool from the trimmer head.
For all previous easy-load bump-feed trimmer heads, there are still the same two (2) forces that will determine how the trimmer line is discharged from the trimmer head, namely (1) the air drag on the line, and (2) the difference in rotational speed between the spool and the housing. However, easy-load type bump-feed trimmer heads generally have two new features not found in other types of bump-feed trimmer heads. Specifically, such trimmer heads have a set of ramps and also have a channel through which trimmer line may pass through. The channel creates a passage way connecting opposite sides of the trimmer head, or at least provides for gripping a strip of line on either side of the spool.
With respect to the ramps, they are placed between the housing and the spool. The user must be able to rotate the spool for the concept to work. In these designs, the user loads the trimmer line into the trimmer head by rotating the bump knob, which is connected to the spool. The vertical wall of the ramp prevents rotation in one direction. The slanted wall of the ramp allows movement in the other direction. It will be appreciated that the trimmer line should be wound in the opposite direction that the trimmer head rotates. The ramp functions well in that the user can rotate the spool in one direction for loading line. But, during this operation, the vertical wall locks the rotation of the spool to the housing until bumped. This eliminates the communication nightmare referenced above and allows loading the line without disassembly of the head.
To make this concept work for trimmers that rotate clockwise and for trimmers that rotate counterclockwise, (i.e., are “universally adaptable”) there must be a means to flip the ramps. To solve this problem, manufacturers of these types of trimmer heads have often added two sets of ramps—one for trimmers that rotate clockwise and another set for trimmers that rotate counterclockwise. Unfortunately, this then creates a new communication nightmare, namely, the consumer now has to be instructed how to select the correct ramps based upon the rotational direction of the trimmer.
Some manufacturers have tried to address this issue by selling two separate trimmer heads—one with ramps oriented for trimmers that rotate clockwise, and a second head with ramps oriented for trimmers that rotate counterclockwise. But this just creates another communications issue in that the consumer must know the rotational direction of their trimmer head beforehand. To complicate issues, the direction of rotation is different depending on whether one defines the rotation from above the trimmer head or from below the trimmer head.
With respect to the line channel, there are patents directed toward line channels that go straight through the center of the spool. Because the line channel goes through the middle of the head, this concept limits the type of trimmers to which it can attach. Basically, it is only commercially viable for an OEM of trimmers with short mounting stems.
One or more other patents teach loading line through a trimmer head with a line channel that goes straight through a flange to the center bore of the internal spool. The line travels around the perimeter of this central opening (bore), and then continues again straight to the far side. These patents are more beneficial for aftermarket trimmer heads allowing connection to a wide range of trimmer models.
There is one easy-load bump-feed trimmer head that works a little differently than those discussed above. This easy-load bump-feed trimmer head is capable of discharging trimmer line based upon air drag on the line alone. This trimmer head has tried to decouple the bump knob from the spool. Thus, the interface between the knob and the spool is two flat surfaces designed to slide upon each other. However, in this design, the bump knob is attached to the drive shaft of the trimmer using a bolt and the bump knob is not free-rotating. There is one bump knob with right-handed threads for trimmers that rotate clockwise and another bump knob with reverse threads for trimmers that rotate in the counterclockwise direction. The design forces the bump knob to rotate at the same speed as the housing, which has been shown to also cause wear to the bump knob. And, even though the design allows the spool to rotate independent of the bump knob, there has been found to be a problem in that there is still friction between the two. Because the spool and bump knob are in contact with each other, there is a force created based upon the coefficient of friction between the two interfacing surfaces, and based upon the force of the spring which pushes the spool against the bump knob. This resulting force based upon friction actually works against the force generated by air drag. The amount of force generated by air drag will vary with the drag characteristics of the line (e.g., its shape, length and stiffness). It has also been found that the head does not perform well if the indexing tabs on the spool have to move up the ramps. The head performs better if the indexing tabs are adjacent to the vertical portion of the ramp and bumping action allows movement over and down the ramps.
Yet another problem with this embodiment of the easy-load trimmer head discussed above is that, as the head is used, dirt and grass clippings can collect between the bump knob and the spool, which is open to the ground. Over time, the interface between the spool and the knob will become abraded. This, in turn, will increase the coefficient of friction, which will increase the resistance to the spool movement and make indexing line out of the head more difficult.
Others have attempted to design a bump-activated trimmer head with a free-spinning bump knob which will releases line based solely upon air-drag on the line. However, these other designs are not an easy-load trimmer head. That is, the trimmer head must be disassembled and the spool removed for winding line onto the spool. Then the consumer must reassemble the head while placing the two free ends of line in the slotted eyelets. Additionally, the fastener cannot be easily changed because it is installed before the free-spinning bump knob is assembled. Changing the fastener requires a complex disassembly of the head beyond what is required to replace the line.
Thus, there is a need in the trimmer head industry for a universally adaptable, easy-load bump-activated trimmer head with a free rotating bump knob that functions excellent in both the clockwise and counterclockwise direction. That is, such a trimmer head would ideally be “universally adaptable,” utilize air drag as the sole means to incrementally extend the stored trimmer line from the trimmer head when the unit is bump-activated, and be loadable without disassembly of the trimmer head. Such a trimmer head would not create friction between the spool and the bump knob, resulting in a force of friction that works against the air drag created by the trimmer line. And, if need be, the desired trimmer head can easily be disassembled for installing an alternate fastener. Still further, such a trimmer head ideally may have a knob for winding the line onto the internal spool which only functions in one direction due to one set of ramps, thereby eliminating any communication nightmares. The trimmer line should be easily indexed out of the head released regardless of the direction of line rotation.